The present invention relates to apparatus and method for detection of secondary charged particles applicable to a low vacuum scanning electron microscope (VP-SEM). More particularly, the present invention belongs to apparatus and method in which electrons having information about an observed surface are amplified through their ionization scattering with gas molecules (gas amplification) and at the same time, amplified ions are transferred toward a detection electrode, so that a positive induced current flowing through the detection electrode is detected to form an image.
The VP-SEM based on the detection principle of gas amplification stands for an electron beam apparatus for forming an image by detecting secondary charged particles attributable to electrons given off to the low vacuum atmosphere under irradiation of a primary electron beam and differs from a normal high-vacuum SEM in detection principle and apparatus construction.
Secondary charged particles generated under irradiation of a primary electron beam are accelerated by an electric field and repeat their ionization scattering with residual gas molecules. Because of occurrence of the repetitious ionization scattering, electrons and ions are amplified (gas-amplified) and by detecting the thus amplified ions/electrons at the electron electrode, an image can be formed. The aforementioned ions/electrons can be detected in the form of positive/negative induced currents flowing through the detection electrode. In order to transfer the generated ions/electrons toward the detection electrode, a potential gradient is set up between the detection electrode and a location where ions/electrons develop.
A scheme for detection of positive induced currents based on ion transfer and that for detection of negative induced currents based on electron transfer are quite the same in physical principle with the exception that the potential gradient supplied inside a specimen chamber and the shape of electrode for electric field supply differ for the respective schemes and there are various forms of detectors in accordance with utilization purposes.
For example, JP-A-2001-126655 discloses a VP-SEM using a positive induced current detection system in which a positive ion current detection electrode is mounted to a specimen stage and a secondary electron collector electrode (connected to a high vacuum secondary electron detector) is arranged around an objective lens so as to be used as an electric field supply electrode. Further, JP-A-2003-132830 discloses a VP-SEM having an ion current detection electrode in a curved surface form arranged between an electric field supply electrode and a specimen stage (on a path on which ionization scattering occurs).
As a negative induced current detection system, a system by G. D. Danilatos et al, for example, disclosed in Scanning 3, 215 (1980) is available. In this scheme, an electrode is used in common as electron current detection electrode and electric field supply electrode and potential on the surface of the detection electrode is maintained to be higher than surrounding potential.
Any of the techniques disclosed in the individual prior art references described as above is for detecting ions/electrons attributable to secondary electrons to thereby obtain a low vacuum secondary electron image. But there is a need for observing not only the low vacuum secondary electron image but also a low vacuum backscattering electron image. The secondary electron image is abundant in edge information and is excellent for observation of a surface structure of a specimen. On the other hand, the backscattering electron image is abundant in composition information and is excellent for observation of a composition distribution in a specimen. Accordingly, the advent of a VP-SEM capable of acquiring both the low vacuum secondary electron image and the low vacuum backscattering electron image has been desired.
JP-A-2002-516647 discloses a VP-SEM capable of detecting both the low vacuum secondary electrons and the low vacuum backscattering electrons by using a negative induced current detection scheme. The construction of the VP-SEM disclosed in JP-A-2002-516647 will be described with reference to FIG. 19.
A specimen 102 to be observed is placed in a specimen chamber 101 maintained in a low vacuum pressure atmospheric environment. Above the specimen 102, a plate-shaped or mesh-shaped detection electrode 103 is arranged and further above, a member 104 used in common as reflection plate and electric field supply electrode is arranged. The detection electrode 103 is at ground potential and the reflection plate/electric field supply electrode 104 is applied with a negative voltage of −100V to −500V from a power supply 105.
For SEM observation, a primary electron beam 106 is focused on an observing surface of the specimen 102 by means of an objective lens 107. From a site subject to irradiation of the primary electron beam 106, backscattering electrons 108 and secondary electrons 109 are generated. The secondary electrons 109 having low energy are absorbed by gas molecules or returned to the specimen stage 110 in a region between specimen stage 110 and detection electrode 103 where no potential gradient exists, thus failing to reach the detection electrode 103. On the other hand, the backscattering electrons 108 having high energy impinge on the reflection plate/electric field supply electrode 104 to generate electrons additionally. The thus generated electrons will hereinafter be called subsidiary electrons 111.
The subsidiary electrons 111 are affected by a potential gradient developing in a region between reflection plate/electric field supply electrode 104 and detection electrode 103 so as to be accelerated toward the detection electrode 103. In this process, the subsidiary electrons undergo ionization scattering with gas molecules and electron-ion pairs are newly created. As the subsidiary electrons approach the detection electrode 103, the ionization scattering process develops exponentially and as a result, electrons are amplified. This phenomenon is called gas amplification. With the amplified electrons approaching the detection electrode 103, negative induced current 112 flows through the detection electrode 103. The induced current 112 is amplified by means of an amplifier 113. The primary electron beam 106 is scanned two-dimensionally on the surface of specimen 102 by using a deflector not shown and signals amplified in synchronism with the scanning are displayed on an image processing terminal not shown, thereby providing a low vacuum backscattering electron image.
Then, disclosed in JP-A-2002-516647 is an example of construction according to which a disk-shaped electrode is used in substitution for the mesh electrode and voltage applied to the aforementioned reflection plate and voltage applied to the disk-shaped detection electrode are adjusted, thereby ensuring that the secondary charged particle detection mode can be switched over (among the mode of detection of only secondary electrons, the mode of detection of only backscattering electrons and the mode of detecting both the secondary electrons and the backscattering electrons). This latter configuration will be described with reference to FIG. 20. In this type of configuration, a disk-shaped electrode 114 in the form of a doughnut is used as the detection electrode, independent or separate power supplies 115 and 116 are connected to the detection electrode 114 and the reflection plate/electric field supply electrode 104 so as to be supplied with voltages V1 and V2, respectively. In case secondary electrons 109 at low energy are detected, V1=V2=+500 V is applied. In this phase, the secondary electrons 109 are accelerated toward the detection electrode 114, undergoing gas amplification. At the detection electrode 114, negative induced current 112 can be detected. For detection of coexistent secondary electrons 109 and backscattering electrons 108, the voltage V1 applied to the detection electrode 114 is fixed to a positive value while the voltage V2 applied to the reflection plate/electric field supply electrode 104 being set to a desired value between −V1 and +V1. To this end, however, an amplifier 117 needs to be a floating amplifier.
With the construction of JP-A-2002-516647, the single VP-SEM can be materialized which can by itself acquire both the low vacuum secondary electron image and the low vacuum backscattering electron image.
In the VP-SEM disclosed in JP-A-2002-516647, the secondary electrons 109 and subsidiary electrons 111 originating from backscattering electrons 108 are detected with the same detection electrode 114 and hence they cannot be detected separately or discriminatingly. Accordingly, in the VP-SEM disclosed in JP-A-2005-516647, both the low vacuum secondary electron image and the low vacuum backscattering electron image cannot be detected at a time.